1. Field of the Invention
This invention relates to tube mills of the multicompartment type for drying and grinding granular material such as cement clinker.
2. Description of the Prior Art
Multi-compartment mills for drying and grinding moist material such as cement clinker have been known to include a separate drying chamber at the inlet for introducing moist material and drying gas. The hot drying gas is usually waste gas from a burning process or it may be air heated for the particular purpose of drying the moist material. The separate drying chamber is usually equipped with lifters or stirrers which improve the drying process. The lifters or stirrers disintegrate and lift the moist material into the hot gas to obtain an intimate mixing and heat exchange therein. Further, it is known in the art to include a modest charge of grinding media in the drying chamber to grind the moist material and thereby increase the surface area for an improved heat exchange.
Such prior art tube mills are generally arranged for a straight (once through) drying and grinding process or for a circuit (material returns to the tube mill) drying and grinding process. In the latter case, a special type of tube mill having feed inlet openings at both ends of the mill and a central discharge compartment is especially applicable.
In the circuit drying tube mills, the drying gas and the moist material are fed at one end of the mill to a drying chamber having (or not having) a charge of grinding media therein. The gas and material proceed from the drying chamber to a coarse grinding chamber from which they pass to the central discharge compartment and out of the mill to a separator. Oversized material may be returned to the mill (preferably to the opposite end of the mill) and into a fine grinding chamber. Fine grinding takes place in the fine grinding chamber (or chambers) and the resulting fine product is likewise discharged to the central discharge compartment of the mill. Drying gas may also be introduced into the fine grinding section of the mill for additional drying of the material.
The prior art sieving diaphragms were constructed and positioned to form end walls of the discharge compartment so as to retain the grinding media and/or material within its respective chamber. When the discharge compartment is so bounded, it becomes difficult to achieve sufficient drying of the moist material because the sieve openings in the sieving diaphragms restrict the passage of the hot gas through the chamber. This limits the amount of gas which can be pressed or drawn through the mill and makes it difficult to have a sufficient amount of the gas pass to the discharge compartment at an acceptable pressure loss. Further, since the sieving diaphragms are circular and segmented, the sieves necessarily include stiffening rings and radial bars. These rings and bars restrict the sieving area of the sieving diaphragms and thereby restrict the passage of the hot gas therethrough.
I have invented a tube mill which avoids these drawbacks and provides an improved approach for drying and grinding granular raw material such as cement clinker.